As for a method of adhering various plastic films to each other, a method of adhering a plastic film to a plastic film on which a metal is deposited, or a method of adhering a plastic film to a thin metal film, a dry lamination method has been conventionally employed. According to the dry lamination method, an adhesive diluted in a solvent such as an organic solvent or water, etc. is applied to a substrate, the solvent is volatilized using a dryer, and the resulting substrate is adhered to another substrate while applying heat and pressure thereto.
As for the adhesives that are used for the dry lamination method, organic solvent-based two-component curing type adhesives are widely used. For example, a first component contains a solution in which a resin comprises a hydroxy group such as polyester resin or polyurethane resin, etc. and a second component contains a polyisocyanate compound.
The organic solvent-based adhesives described above contain a great amount of an organic solvent. As such, a dry lamination method using the adhesives has various problems such as environmental contamination due to exhaust gas, risk of fire explosion, and sanitary problems in working places, etc. Since the conventional organic solvent-based adhesive contains a solid matter, the content of which is 30% by weight or less, a great amount of organic solvent is released to the outside atmosphere when the organic solvent is volatilized after application of the adhesive. As such, high-solid type adhesives having a high solid content in which the amount of an organic solvent is reduced have been suggested (See, Patent Document No. 1).
However, when such high-solid type adhesives are used for a dry lamination method which is based on a conventional gravure process, a stripe pattern is formed in the direction of coating flow on the surface of a laminate after lamination. For such a reason, there has been a problem in that uniform appearance is not obtained and good finishing results are not easily obtained. In addition, so-called tunneling phenomenon sometimes occurs by which a laminate obtained after lamination is peeled off in a tunnel shape.
Recently, in view of demands to improve condition for working environment, to strengthen fire regulations and to restrict release of VOC (volatile organic compound) to atmosphere etc., nonorganic solvent-based adhesives are strongly required. As such, a dry lamination method using water-based adhesives in which the adhesives are solubilized in water has been suggested. According to the dry lamination method using water-based adhesives, working environment can be improved and problems associated with fire control and release of VOC to atmospheric air, etc. can be solved. However, a new problem which is derived from its intrinsic aqueous characteristic arises. Specifically, due to poor wetting into a plastic substrate which is attributing to high surface tension of water, a stripe pattern is formed on the surface of a laminate obtained after lamination, and therefore there is a problem in that good appearance cannot be obtained.